Conventionally, in wireless communication systems that perform packet wireless transmission such as wireless LAN systems, the access control method based on carrier sense (i.e. CSMA method) has been commonly used. For example, Non-patent Document 1 discloses a CSMA/CA method that uses, in addition to the carrier sense based on received-signal strengths, a request-to-send (RTS) packet and a clear-to-send (CTS) packet.
The CSMA/CA method is aimed to avoid packet collisions due to so-called hidden nodes, which cannot be detected by a node but interferes with a communication target of the node.
However, in the CSMA method and the CSMA/CA method, a node that has detected an interference signal by means of the carrier sense function thereof cancels data transmission even though the received-signal strength of the interference signal is so weak that the communication is not obstructed. This is called an “exposed-node problem”. Because of this problem, the system as a whole is limited in the transmission capacity.
FIG. 20 is a conceptual system diagram for explaining operations performed through a conventional CSMA method. FIG. 21 is a time sequence diagram showing packet transmissions performed in the system shown in FIG. 20.
In FIG. 20, a wireless station 101 transmits data to a wireless station 102 and a wireless station 103 transmits data to a wireless station 104 based on the CSMA method.
In FIG. 21, the wireless station 103 firstly performs the carrier sense (CS) to start transmission of data, and after confirming that no carrier has been detected, transmits the data to the wireless station 104. Here, if the wireless station 101 attempts to start the transmission of data while the wireless station 103 is transmitting data, the wireless station 101 detects a signal of a data packet transmitted by the wireless station 103, using the carrier sense (CS). Accordingly, the transmission performed by the wireless station 101 will be prohibited. On the other hand, if the wireless station 103 attempts to start the transmission while the wireless station 101 is transmitting data, the wireless station 103 detects a signal from the wireless station 101, using the carrier sense. Accordingly, the transmission performed by the wireless station 103 will be prohibited. With conventional CSMA methods, such a transmission prohibition status is caused even if the strength of the interference signal, transmitted by the wireless station 103 and received by the wireless station 102, is sufficiently lower than the desired signal transmitted by the wireless station 101 and does not obstruct the reception of the desired signal. As a result, it is impossible to improve the transmission capacity of the system as a whole.
To solve this problem, Patent Document 1 discloses a technique wherein the access point discriminates between interference areas and non-interference areas, and allocates different communication periods to wireless stations existing within the interference areas and wireless stations existing in the non-interference areas. Here, the interference areas are, in terms of a wireless station existing therein, areas in which the carrier-to-interference ratio (CIR) between a desired signal transmitted from an access point as a communication target and an interference signal transmitted from another access point is less than the lower limit of the CIR required for establishment of communication (required CIR). The non-interference areas are, in terms of a wireless station existing therein, areas in which the carrier-to-interference ratio (CIR) between a desired signal transmitted from an access point as a communication target and an interference signal transmitted from another access point is equal to or more than the required CIR.
FIG. 22 is a conceptual diagram showing a wireless communication system using the technique disclosed by the above-mentioned Patent Document 1. In FIG. 22, the reference number 200 represents a control station, 201 and 202 represent access points, and 203-206 represent wireless stations. Here, the wireless stations 203 and 205 are communicating with the access point 201, and the wireless stations 204 and 206 are communicating with the access point 202. Each access point judges whether each wireless station exists within the interference area or not, based on the received power from each wireless station, by means of a prescribed procedure using beacon and so on.
In the example of FIG. 22, the wireless station 203 and the wireless station 204 exist in the non-interference areas corresponding to different access points respectively, and both of them can secure the CIR greater than the required CIR even if they perform communications at the same time. Meanwhile, the wireless station 205 and the wireless station 206 exist in the interference areas, and they cannot perform communications at the same time. The wireless stations 205 and 206 existing in the interference areas perform time-division communications in the period allocated for the interference areas, using the CSMA in the conventional manner. The wireless stations 203 and 204 existing in the non-interference areas perform communications in the period exclusively allocated for the non-interference areas without performing the carrier sense. As a result, the wireless stations belonging to the different access points can perform communications at the same time in the period exclusively allocated for the non-interference area without causing packet loss. This improves the transmission capacity of the system as a whole.
The Non-patent Document 1 also discloses a technique to avoid packet collisions, based on virtual carrier sense that uses the RTS/CTS and reservation time information. FIG. 23 is a time sequence diagram showing packet transmissions in the case where the communication control method using the RTS/CTS and the reservation time information is applied to the system of FIG. 20.
In FIG. 23, firstly the wireless station 103 performs the carrier sense (CS). Not detecting any carrier, the wireless station 103 transmits an RTS packet to the wireless station 104. The RTS packet includes reservation time information to be used for reservation of a time period until completion of transmission of a data packet to be transmitted and an acknowledgement (ACK) packet corresponding to the data packet.
Upon reception of the RTS packet transmitted from the wireless station 103 to the wireless station 104, the wireless station 101 sets a timer of a network allocation vector (NAV) based on the reservation time information included in the RTS packet, and comes into the transmission prohibition status until the end of the reservation time.
If the wireless station 104 is ready to receive packets and not in the transmission prohibition status, the wireless station 104 transmits a CTS packet to the wireless station 103. Upon reception of the CTS packet from the wireless station 104, the wireless station 103 transmits a data packet to the wireless station 104.
In the case of transmitting two or more data packets in succession, each of the data packets except for the last data packet includes reservation time information for reservation of a time period required for completing transmission of the next data packet and the ACK packet corresponding thereto.
FIG. 23 shows the case where the wireless station 103 transmits two data packets in succession. Based on the reservation time information included in the first data packet, the wireless station 101 updates the time of the NAV, and extends the period of the transmission prohibition status until the time when transmission of the next data packet and the ACK packet is expected to be completed. In this way, in the case where the wireless station 102 and the wireless station 103, or the wireless station 101 and the wireless station 104 are at positions where they can not communicate with each other, it is possible to avoid the packet collisions due to transmission performed at the same time.
With this structure, however, the wireless station 101 stays in the transmission prohibition status for a long time if receiving a signal from the wireless station 103. Accordingly, the transmission prohibition status is caused even though the strength of the interference by the wireless station 103 with the wireless station 102 is sufficiently lower than the strength of the signal transmitted from the wireless station 101 to the wireless station 102 and does not obstruct communications. Therefore, it is impossible for this structure to improve the transmission capacity of the system as a whole.
To solve this problem, Patent Document 2 discloses a technique for improving the transmission capacity of the system as a whole, wherein a wireless station judges whether a signal that the wireless station transmits interferes with other wireless stations based on whether the wireless station receives an RTS packet or a CTS packet, and transmits signals at the same time if they do not interfere with each other.
FIG. 24 is an example of a time sequence of the packet transmissions disclosed in the Patent Document 2. In this example, the following procedures are the same as those shown in FIG. 23: the wireless station 103 transmits an RTS packet; correspondingly the wireless station 104 transmits a CTS packet; and the wireless station 103 transmits the data packet. However, the following are different from FIG. 23: the period between when the wireless station 103 has completed transmission of the data packet and when the wireless station 104 starts transmission of an ACK packet is secured such that an RTS packet can interrupt therein.
In the case where the wireless station 101 has received an RTS packet from the wireless station 103 but has not received a CTS packet corresponding to the RTS packet from the wireless station 104, the wireless station 101 judges that the wireless station 101 and the wireless station 104 are located as not interfering with each other. Accordingly, immediately after the wireless station 103 completes transmission of a data packet, the wireless station 101 transmits an RTS packet to the wireless station 102. The wireless station 102, which has received the RTS packet of the wireless station 101, transmits a CTS packet to the wireless station 101 if signals of the wireless station 103 and the wireless station 104 have not been received. Although this transmission of the CTS packet overlaps with the transmission of the ACK packet from the wireless station 104, the wireless station 101 can receive the CTS packet without any problems because the wireless station 101 is at a position where the wireless station 104 does not interfere with. Subsequently, the wireless station 101 transmits a data packet to the wireless station 102. Although this transmission of the data packet overlaps with the transmission of a data packet from the wireless station 103 to the wireless station 104, the both data packets can be transmitted at the same time because the wireless station 104 is at a position where the wireless station 101 does not interfere with, and the wireless station 102 is at a position where the wireless station 103 does not interfere with.    Patent Document 1: Japanese Laid-open Patent Application Publication No. 2004-260637    Patent Document 2: Japanese Laid-open Patent Application Publication No. 2001-345809    Non-Patent Document 1: ANSI/IEEE Std 802.11, 1999 Edition